1. Field of the Invention
The present invention relates to a method for the preparation of the monopotassium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid. More particularly, it relates to a method for the preparation of the monopotassium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid of the following formula: ##STR1## starting from 3,4-dicyano-1,2,5-thiadiazole, a readily obtainable compound, for instance, by cyclization of diaminomaleonitrile (a tetramer of hydrocyanic acid) with S-chlorides, according to methods known in the art.
2. Description of the Prior Art
The monopotassium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid is a known compound which has important and useful applications as an intermediate for the preparation of polyamide polymers for use as effective membranes for reverse osmosis, which membranes are used in the field of sea water desalination. As is well known, said polymers may be obtained, for instance, by the interfacial polymerization of diamines with the dichloride derivative of 1,2,5-thiadiazol-3,4-dicarboxylic acid, which in turn is obtainable by effecting chlorination (with SOCl.sub.2) of the monopotassium salt of 1,2,5-thiadiazol-3,4-dicarboxylic acid, which salt may be obtained according to the method of this invention.
Prior methods for preparing the monopotassium salt of 1,2,5-thiadiazol-3,4-dicarboxylic acid are based on the oxidation of 2,1,3-benzo-thiadiazoles with potassium permanganate. Such methods are, however, unsuitable for industrial use because of the low yields encountered as well as the complexities of operation.
It is also known to prepare the monopotassium salt of the 1,2,5-thiadiazol-3,4-dicarboxylic acid by saponification of 3,4-dicyano-1,2,5-thiadiazole with KOH, according to the known technique of alkaline saponification of nitriles. This method involves the heating, under reflux, of a suspension of the nitrile in dilute aqueous KOH, with a high excess of KOH over the stoichiometric quantity required. We have found, however, that in utilizing this saponification technique with respect to 3,4-dicyano-1,2,5-thiadiazole, the yields do not exceed 85%, and in addition, there occurs the undesirable formation of cyanide ions due to a secondary reaction of the KOH with the 3,4-dicyano-1,2,5-thiadiazole. Moreover, the reaction volume is considerable, so that in order to isolate the reaction product, which exhibits a fair solubility in water, one must resort to considerable concentration of the solution at the end of the reaction.
In addition to the above drawbacks, there is also the fact that there is present in the waste waters considerable quantities of cyanides, with consequent potential damages and environmental problems.